This invention relates to cellulose ethers and a method of preparing the same from mercerized and recovered cellulose pulp. This invention also relates to a method of preparing cellulose floc with increased bulk density from mercerized and recovered cellulose pulp.
Cellulose ethers have widespread application in thickening agents (e.g. in food additives), bonding agents (e.g. in lacquers and other paints), adhesives, printing pastes, suspension stabilizing agents, thermoplastic materials, protective colloids, emulsion stabilizers, finishing compositions (e.g. in textiles), coating compositions (e.g. in paper and paper products), plastic sheets (e.g. in packaging or textiles), and film-forming agents. See, for example, Cellular Materials to Composites, Volume 3, Chapter xe2x80x9cCellulose Ethersxe2x80x9d, pages 226-269, Encyclopedia of Polymer Science and Engineering, 2nd Ed., 1985, John Wiley and Sons, New York. Many of these applications benefit from the relatively high viscosity of cellulose ethers (e.g. petroleum production fluids). Prior to the present invention, the solution viscosity of cellulose ethers have primarily been increased by increasing the degree of polymerization (DP) or viscosity of the cellulose pulp used to make the cellulose ethers, or by protecting against DP or viscosity degradation during pulp grinding and other processes performed during cellulose ether manufacturing.
For example, if a specific solution viscosity is desired for the cellulose ethers, the appropriate cellulose pulp viscosity is determined and a cellulose pulp having this viscosity is selected. This raw material selection strategy is also employed by ether producers to increase production throughput by using higher density cellulose floc.
Cellulose ethers are typically produced by alkalating cellulose with an alkalating agent, such as sodium hydroxide, to form an alkali cellulose and then etherifying the alkali cellulose. See, for example, U.S. Pat. Nos. 2,067,946; 2,636,879; 4,063,018; 4,250,305; 4,339,573; and 4,547,570. The cellulose pulp may be shortened or granulated into a cellulose floc before being alkalated. See, for example, U.S. Pat. Nos. 2,067,946; 2,636,879; and 4,339,573.
Edelman et al., U.S. Pat. No.4,941,943, disclose a pretreatment process for preparing sodium carboxymethyl cellulose. The pretreatment process includes slushing cellulose to a consistency of about 5-15% to form a fibre suspension, concentrating the fibre suspension to a consistency of about 25-35% to form a pulp, and homogenizing the pulp. After the fibre suspension has been concentrated, the pulp is mercerized to form activated cellulose (or alkali cellulose). The mercerization step may occur before, after, or concurrently with the homogenizing step. After the pretreatment process, the activated cellulose is etherified to form the sodium carboxymethyl cellulose.
Orii et al., U.S. Pat. No. 4,292,426, disclose a process for preparing hydroxylpropyl cellulose. The process requires swelling cellulose, driving out excess caustic soda solution from the swollen cellulose to form a dehydrated cellulose, and obtaining an alkali cellulose by washing with an aqueous caustic solution and drying the dehydrated cellulose. The alkali cellulose is washed in order to reduce the alkali content of the dehydrated cellulose. The resulting alkali cellulose has a sodium hydroxide to cellulose ratio of 0.05 to 0.16 and a water to cellulose ratio of 0.2 to 0.5. The process further comprises reacting the alkali cellulose with propylene oxide until the molar substitution is between 1 and 2, adding 0.2 to 0.7 parts of water for every one part of cellulose in the raw material alkali cellulose, and continuing the etherification reaction until the molar substitution is higher than 2.5.
Bredereck et al., U.S. Pat. No. 4,491,661, disclose a process for preparing water-soluble cellulose ethers comprising activating cellulose with ammonia, alkalizing the activated cellulose with an alkalizing agent in the presence of ammonia, removing the ammonia from the alkali cellulose in the presence of the alkalizing agent, and etherifying the alkali cellulose with an etherifying agent in the presence of an organic solvent. When cellulose is activated with liquid ammonia, crystalline cellulose III is formed. As well known in the art, crystalline cellulose III has a significantly different structure and properties than other crystalline cellulose structures, such as crystalline cellulose II. See, for example, Rydholm, supra; and Ott, Spurlin and Grafflin, supra.
Laskowksi et al., DDR Patent No. 146,462, disclose a process for preparing low substituted, water soluble carboxymethyl cellulose having a degree of substitution less than 0.5. The process includes activating cellulose with liquid ammonia, steaming off the ammonia, alkalizing the activating cellulose with sodium hydroxide, and carboxymethylizing the alkalized cellulose.
Dautzenberg et al., DDR Patent No. 148,342, disclose a process for preparing carboxymethyl cellulose with increased solution viscosity. The process includes activating cellulose with liquid ammonia, alkalizing the activated cellulose, and carboxymethylizing the alkalized cellulose.
There is a need for a method of preparing cellulose ethers which increases throughput and in which the solution viscosity of the cellulose ethers may be controlled by process conditions, not just the viscosity of the starting raw materials. There is also a need for a method of preparing cellulose floc which increases the bulk density per number average floc length.
The present inventors have discovered that the solution rheology of cellulose ethers prepared from cellulose pulp is altered by mercerizing and recovering cellulose pulp before preparing the cellulose ethers. For example, the solution viscosity of carboxymethyl cellulose (CMC) produced from mercerized and recovered cellulose pulp is significantly greater than that produced from non-mercerized cellulose pulp.
The present invention provides a method of preparing cellulose ethers comprising the steps of (a) obtaining mercerized and recovered cellulose pulp, and (b) converting the mercerized and recovered cellulose pulp into the cellulose ethers. According to one embodiment, when the cellulose pulp is southern softwood kraft, the mercerized cellulose pulp has a TAPPI 230 om-89 viscosity greater than 12 cP. This method, however, may be applied to all cellulose pulps, regardless of their viscosities, including those which, when mercerized, have a viscosity less than 12 cP. The mercerized cellulose pulp is typically substantially free of cellulose III. Mercerized cellulose pulp prepared by this method has a greater percentage of crystalline cellulose II and a smaller crystalline area than that of non-mercerized cellulose pulp.
According to a preferred embodiment, the method comprises the steps of (a) obtaining mercerized and recovered cellulose pulp, (b) treating the mercerized and recovered cellulose pulp to form a cellulose floc, (c) alkalating the cellulose floc to form an alkali cellulose, and (d) etherifying the alkali cellulose to form a cellulose ether. According to an alternative embodiment, steps (a) and (b) are reversed, i.e., the cellulose pulp is treated to form a cellulose floc and then the cellulose floc is mercerized and recovered.
The present invention also provides a method of preparing a cellulose floc comprising the steps of (a) obtaining mercerized and recovered cellulose pulp, and (b) treating the mercerized pulp to form the cellulose floc. According to one embodiment, when the cellulose pulp is southern softwood kraft, the mercerized cellulose pulp has a TAPPI 230 om-89 viscosity greater than 12 cP. This method, however, may be applied to all cellulose pulps, regardless of their viscosities, including those which, when mercerized, have a viscosity less than 12 cP. Alternatively, the method comprises mercerizing and recovering a cellulose floc. Cellulose floc prepared by this method have a greater bulk density than cellulose floc prepared from similar non-mercerized cellulose pulp. Furthermore, the bulk density gain is greater than that expected from the coarseness (weight per unit of fiber length) gain from preparing a cellulose floc.
Yet another embodiment is a method of preparing cellulose ethers comprising the steps of (a) selecting a desired viscosity for the cellulose ethers, (b) obtaining a mercerized and recovered cellulose pulp having the appropriate viscosity for yielding cellulose ethers having the selected viscosity, and (c) converting the mercerized and recovered cellulose pulp to the cellulose ethers. According to one embodiment, when the cellulose pulp is southern softwood kraft, the mercerized cellulose pulp typically has a TAPPI 230 om-89 viscosity greater than 12 cP. This method, however, may be applied to all cellulose pulps, regardless of their viscosities, including those which, when mercerized, have a viscosity less than 12 cP.